Communication systems, and particularly cellular communication systems, are well known. Cellular communication systems, as are known, consist of individual cell sites, each equipped to communicate with mobile communication units operating within the cell site. Communication in a cell site is conducted over a communication resource, commonly referred to as a communication channel, which may consist of a pair of radio frequencies used by the mobile communication unit to transmit and receive information with the cell site. More recently, digital cellular communication systems have been developed in which the communication channel consists of a series of time slots or packets that contain frames of information communicated between the mobile communication unit and the base station. These digital communication systems have greatly enhanced the capacity of the communication system as multiple users may be allocated to a single communication channel by multiplexing the frames of information onto the communication channel.
In digital communication systems, and for example a digital cellular communication system, a speaker's voice is first converted to a modulated signal. This modulated signal is then encoded, i.e., transformed from the modulated signal to a digital representation of the modulated signal which is suitable for transmission on the communication channel. The encoded signal is transmitted to a receiving unit, e.g., a mobile communication unit, where it is decoded, demodulated and reproduced as an audio representation of the speaker's voice.
In order to control the communication activity within the cellular communication system, control information or signaling, is communicated between the mobile communication units and the base station controller at the cell site. In, for example, a time division multiple access (TDMA) system, a particular time slot of the information frame can be dedicated to carrying the signaling information. This arrangement, however, can prove to be inefficient in that the time slot is always reserved for signaling even when signaling is not required. By reserving this time slot, the available bandwidth for carrying information, such as encoded speech, is reduced.
In code division multiple access (CDMA) type communication systems, no particular portion of the information frame is reserved for signaling. Each information frame is therefore efficiently used to carry a maximum amount of information if required. It is also known in CDMA systems to provide variable rate communication. Variable rate communication provides, in some instances, for less than the full information frame to be used to communicate a particular user's information. The remaining portion of the information frame is thus available to service other users, or, as will be described, to communicate signaling.
With variable rate communication, when a user is speaking the information is typically communicated at full-rate. That is, the entire information frame is dedicated to carrying the user's encoded speech. However, during pauses in speech, full-rate communication is not required to effectively communicate all of the speaker's information, and hence, lower rates such as half (1/2) rate, quarter (1/4) rate or eighth (1/8) rate are used. In half-rate, the user's information fills only 1/2 of the information frame or information bandwidth. Information from another user of the system can therefore be inserted into the frame, or the remaining portion of the information bandwidth, and communicated over the communication channel.
When signaling is required, i.e., signaling between, for example, the mobile communication unit and the base station, or vice versa, the signaling device will attempt to wait for a less than full-rate communication frame into which to insert the signaling information. However, the signaling information typically includes a "time-out" or time period in which it must be sent. In the event that the time-out is reached without the signaling information being sent, the signaling device will insert the information in what is known as either a "blank-and-burst" or "dim-and-burst" process.
During a blank-and-burst, an entire frame or more of encoded speech is discarded and the signaling information is inserted in place of the encoded speech. In a dim-and-burst, the speech is encoded at less than full-rate, such as 1/2 rate, and the signaling information is inserted in with the frame with reduced information bandwidth. During blank-and-burst or dim-and-burst operations, a portion of the encoded speech is lost as signaling is transmitted in its place resulting in some audio disruption. Before conducting a blank-and-burst or dim-and-burst, however, the speech encoder at the signaling device adjusts its state accordingly so that the receiving device decoder does not significantly diverge as a result of the lost speech bandwidth and audio quality is substantially maintained.
In a mobile-to-mobile communication, the speech is encoded at the transmitting mobile communication unit, communicated through the cellular communication network and received by the receiving mobile where it is decoded. While being communicated through the cellular communication network, the speech is not decoded, but it is merely communicated to its destination. If, however, the cellular communication network has to transmit signaling information to the receiving mobile, the only way to do so is by conducting a blank-and-burst. This can lead to excessive divergence between the encoder at the transmitting mobile and the decoder at the receiving mobile communication unit. As discussed, when inserting signaling information, the signaling device encoder is normally allowed to adjust its state such that divergence between the decoder and the encoder is avoided. However, when the signaling information is inserted by the cellular network equipment in a mobile-to-mobile communication, the cellular network can not inform the speech encoder of the mobile communication unit to adjust its state. The decoder, anticipating frames containing encoded speech diverges from the encoder causing a reduction in the audio quality.
Disruptions of the frames during communication is not uncommon and the decoder is adapted to treat the loss of speech frames (known as frame erasures) due to corruption. To account for insertion of signaling by the cellular communication network in a mobile-to-mobile communication, it has been proposed to allow the decoder to simply consider the blank-and-burst signaling frames as frame erasures such as if the frame were corrupted during transmission. Such a method suffers a number of disadvantages. During initial call set-up of a mobile-to-mobile communication significant additional signaling and processing is required. During this processing it is also required that the audio signal be muted for up to one second so that all of the required call set-up signaling may be communicated to the mobile communication units, which includes informing the mobile communication units that the call is a mobile-to-mobile call. The mobile communication units then consider blank-and-burst signals as frame erasures during that particular call. Testing, however, reveals that with this method there is only marginal improvement in audio quality as compared to making no accommodation for the blank-and-burst frames.
Therefore, there is a need for enhancing audio quality during signaling insertion in a cellular communication system, and particularly, in the mobile-to-mobile communication scenario.